1. INTRODUCTION

With the discovery of the Cosmic Background Radiation by Penzias
and Wilson (1965), cosmology became a branch of physics: there was
a well defined framework within which to formulate models and
confront them with observational data. Prior to that there had
been a few important observations and a few important solutions to
the Einstein Field Equations for General Relativity. We suspected
that these were somehow connected: that the Friedman-Lemaitre
solutions of the Einstein field equations described the
cosmological redshift law discovered by Hubble.

With the discovery of the background radiation we were left in no
doubt that the Universe had a hot singular origin a finite time in
our past. That important discovery also showed that our Universe,
in the large, was both homogeneous and isotropic, and it also
showed the appropriateness of the Friedman-Lemaitre solutions.

The establishment of the "Big Bang" paradigm led to a search for
answers, in terms of known physical laws, to key questions: why
was the Universe so isotropic, how did the structure we observe
originate? and so on. Cosmologists built models involving only
known physics and confronted them with the data. Cosmology became
a branch of physics with a slight difference: we cannot experiment
with the subject of our discussion, the Universe, we can only
observe it and model it.

With the current round of cosmic microwave background anisotropy
maps we are able to see directly the initial conditions for galaxy
formation and for the formation of large-scale structure. That
observed structure is thought to reflect directly the fluctuations
in the gravitational potential that gave birth to cosmic structure
and it is a consequence of the physics of the early universe. The
goal is to link those initial conditions with what we see today.

The aim of this article is to show how the "homogeneous and
isotropic Universe with a hot singular origin" paradigm has
emerged, and to explain how, within this framework, we can
quantify and understand the growth of the large scale cosmic structure.